Series hybrids do have efficiency advantages in some situations -- for example, they can run in EV mode at any speed, and they can also have more freedom to run the engine at speeds independent of the vehicle speed, which can enable it to be programmed to prefer running it at speeds dictated by efficiency rather than by the drive wheel speed.

The place where they fall behind is at steady state on the highway -- but all of the series hybrid systems on the road have a solution for this problem too! They typically have clutch that engages a one-speed direct drive from the engine to the wheels. This skips the double-conversion losses at highway cruising. Then if you give it some gas to accelerate, the clutch disengages and you go back to full double-conversion again.